MirOS BSD (originally called MirBSD) is a free and open source operating system which started as a fork of OpenBSD 3.1 in August 2002.

Since then it has also incorporated code from other free BSD descendants, including NetBSD, MicroBSD and FreeBSD.

According to the developers, "controversial decisions are often made differently from OpenBSD; for instance, there won't be any support for SMP in MirOS".

There will also be a more tolerant software inclusion policy, and "the end result is, hopefully, a more refined BSD experience".

MirLibtool was a modified version of GNU libtool 1.5 installed by MirPorts to build shared libraries in a portable way.